This invention relates to a process of producing polytrimethylene terephthalate by esterification of terephthalic acid with 1,3-propane diol, precondensation of the esterification product to obtain a precondensation product, and polycondensation of the precondensation product to obtain polytrimethylene terephthalate. More specifically, the present invention relates to an improvement in said process wherein byproduct cyclic dimer of 1,3-propane diol and terephthalic acid is removed from the polymerization mixture, converted into polymerizable monomers, and recycled into the polymerization mixture.
The preparation of polytrimethylene terephthalate (PTT) involves the reaction of terephthalic acid (TPA) or dimethylterephthalate (DMT) and excess 1,3-propane diol (PDO) at elevated temperatures, 240 to 275xc2x0 C., optionally in the presence of an esterification catalyst such as a catalytic titanium compound, to obtain an esterification product which is usually a relatively low intrinsic viscosity PTT. This esterification product is then subjected to precondensation and finally the precondensation product is subjected to polycondensation to obtain PTT. In some processes, this is followed by solid state polymerization to increase the intrinsic viscosity of the PTT but there is a new process which can produce high intrinsic viscosity PTT without solid state polymerization.
There are several byproducts which are produced by this polymerization reaction. One of these byproducts is the cyclic dimer of PDO and TPA or DMT. This PTT cyclic dimer shown below 
is only slightly soluble, if at all, in PDO at temperatures below about 175xc2x0 C. Thus, unless the solution is held at 175xc2x0 C. or above, this cyclic dimer will precipitate when the solution is cooled and accumulate as a solid in the polymerization process equipment requiring removal. Handling such solids in a process is expensive. Also, if the temperature is maintained at an elevated level, handling hot liquids increases the cost of manufacturing the product. The cyclic dimer is so volatile that it comes out of the polymer during polymerization. The process usually incorporates the step of recovering the cyclic dimer during one of the vacuum stages of the polymerization process, i.e., in precondensation or polycondensation or even in solid stating. The cyclic dimer is always in equilibrium with the polymer at elevated temperatures (about 2.5 wt % during polycondensation and about 1 wt % during solid state polymerization which is carried out at somewhat lower temperatures), so it will be replaced but this prevents it from accumulating in the polymer in the melt or solid-state reactors. However, this also causes the loss of the cyclic dimer material in the process equipment, which losses can be up to 1% or more of the total weight of polymer in the reactor. The cyclic dimer is typically recovered as a solid or as a suspension or slurry in 1,3-propanediol.
Thus, it can be seen that it would be advantageous to find a way to recycle the cyclic dimer so that the lines carrying PDO with cyclic dimer do not have to be heated to 175xc2x0 C. or above and so that the cyclic dimer could be converted into PTT to enhance the overall yield of the process. The present invention provides such a process.
This invention is an improvement upon the known process for polymerization of PTT by esterification of TPA or DMT with PDO, precondensation of the esterification product to produce a precondensation product, and polycondensation of the precondensation product to produce PTT and, optionally, solid stating of this product to produce higher intrinsic viscosity PTT. The cyclic dimer of TPA or DMT and PDO is produced as a byproduct of the above process and the improvement comprises:
(a) recovering the cyclic dimer from the polymerization,
(b) reacting from 0.5 to 25 percent by weight of the cyclic dimer in PDO in the presence of from 0.1 to 10 percent by weight of the cyclic dimer of a basic catalyst for five minutes to 24 hours at 50 to 210xc2x0 C.,
(c) optionally neutralizing the basic catalyst in the reaction product of (b), and
(d) recycling the reaction product into the polymerization process.
PTT can be prepared by reacting 1,3-propane diol (PDO) and terephthalic acid (TPA) or dimethylterephthalate (DMT) optionally including other diols and/or aromatic diacids or diesters thereof, with removal of byproduct water (or alcohol), for a time effective to produce a polyester having an intrinsic viscosity of at least about 0.6 dl/g as measured in 60/40 phenol/tetrachloroethane at 25xc2x0 C. In one variation of this process, a PDO based polyester such as PTT can be prepared in a two-stage condensation polymerization process. The first stage, melt polycondensation or esterification, includes two steps, a xe2x80x9cpressure stepxe2x80x9d followed by a xe2x80x9cvacuum step.xe2x80x9d In the pressure step, a molar excess of PDO is reacted with the diacid or alkyl ester thereof, optionally in the presence of added catalyst (an esterification catalyst such as a transition metal catalyst, especially titanium or tin can be used) at a temperature within the range 240 to 275xc2x0 C. under atmospheric or superatmospheric pressure. Water or alcohol is produced as a byproduct and is removed by suitable means such as overhead distillation. The polymerization conditions are selected so as to produce a relatively low molecular weight polyester having an intrinsic viscosity of less than about 0.3, usually within the range of about 0.05 to about 0.25 dl/g.
For the vacuum step of the melt polycondensation, the pressure on the reaction mixture is reduced and a catalyst is usually added. The preferred polycondensation catalysts are compounds of titanium or tin, such a titanium butoxide, present in an amount within the range of about 10 to about 400 ppm titanium or tin, based on the weight of the polymer. This step is commonly divided into the precondensation stage and the polycondensation stage, mainly as a way to let the pressure down gradually. The low molecular weight product of the first step is heated at a temperature within the range of about 240 to about 275xc2x0 C. under less than atmospheric pressure for a time effective to increase the intrinsic viscosity of the starting material to at least about 0.5 dl/g. During the reaction, additional water or alcohol is produced as a byproduct and is removed overhead along with excess diol. The cyclic dimer byproduct can also be removed in the overhead at this point in the reaction.
The reaction product of the melt stage is cooled, solidified, and optionally formed into pellets. The polymer can then be polycondensed in solid form (xe2x80x9csolid-statedxe2x80x9d) at an elevated temperature less than the target polymer melt point, generally (for PTT) at a temperature greater than about 180xc2x0 C. and preferably above about 200xc2x0 C., under reduced pressure and/or an inert gas stream. The solid stating phase is carried out for a time, generally about four hours or more, sufficient to produce a polyester having an intrinsic viscosity of at least about 0.8, generally within the range of about 0.9 to about 1.1 dl/g.
There is also a new continuous all melt process for producing PTT which does not require the solid stating step to reach high intrinsic viscosity. This process is described in copending application Ser. No. 09/840,411, filed Apr. 23, 2001, which is herein incorporated by reference.
The cyclic dimer described above is formed as a byproduct of the reaction of PDO and TPA or DMT. The cyclic dimer occurs at levels of about 2.5 percent by weight in the melt polymer from the polycondensation step and about 1 percent by weight in the final solid stated polymer. At typical polycondensation temperatures below about 275xc2x0 C., the cyclic dimer content of the PTT melt is always less than 3.0 weight percent and it""s concentration is unaffected by catalysts or other additives. Unlike cyclic oligomers formed in the production of polyethylene terephthalate, i.e., polyethylene terephthalate cyclic trimer, the PTT cyclic dimer is so volatile that a significant amount can be recovered during the vacuum step of the PTT polymerization process as described above and also during the solid state polymerization under vacuum or nitrogen flow.
Although the cyclic dimer will dissolve to some extent in PDO at elevated temperatures, e.g., about 175xc2x0 C. or above, it will precipitate from PDO when the solution is cooled. After it is recovered from the polymerization mixture, the cyclic dimer mixed with PDO is treated with a small amount of a basic catalyst. The basic catalyst is preferably an alkali or alkaline earth metal basic salt, such as hydroxide, carbonate, bicarbonate, and the like. Carboxylate salts, such as acetates and terephthalates also can be used as well as alkoxides, such as methoxides or ethoxides. Sodium, potassium, rubidium, magnesium, calcium, and strontium bases are preferred, most particularly sodium and potassium hydroxides and carbonates.
The amount of basic catalyst used is from 0.1 percent to 10 percent by weight, preferably from 1 to 5 percent by weight, based on the weight of the cyclic dimer. Although higher amounts of basic catalyst can be used to increase the rate of reaction of cyclic dimer in PDO, excessive amounts of basic catalysts can also interfere with the subsequent use of the PDO solution in preparing PTT polymer.
The cyclic dimer mixture in PDO generally contains from about 0.5 to 25 percent, preferably from 1 to 10 percent of the cyclic dimer by weight of the initial reaction mixture. In either case, the final reaction mixture product can be combined with additional PDO, if desired, before being added back to the polymerization process.
The reaction of the cyclic dimer with PDO in the presence of the basic catalyst is conducted at temperatures from 50 to about 210xc2x0 C., preferably from 80 to 180xc2x0 C., and most preferably from 100 to 150xc2x0 C. The reaction time will depend upon the temperature used but it can be as little as five minutes up to 24 hours or more. Typically, the reaction time ranges from 10 to 150 minutes at the preferred temperatures. When the reaction mixture is cooled to room temperature, a very small amount of precipitate, typically less than about 10 percent of the cyclic dimer charged, may be formed. This precipitate can be easily removed, if desired, by filtration.
Based on NMR analysis, we believe that most of the cyclic dimer is converted into linear diesters such as
HOCH2CH2CH2(Oxe2x80x94COC6H4COxe2x80x94OCH2CH2CH2)xOH 
where x is 1 or 2, which are more soluble in PDO. This reaction mixture can then be used, in whole or in part, for the preparation of high molecular weight PTT. If desired, the basic catalyst may be partially or completely neutralized prior to subsequent preparation of PTT by addition of a non-oxidizing acid such as phosphoric acid, organosulfonic acid, hydrochloric acid, and the like, or by passing the solution through an acidic ion exchange resin. However, under the preferred conditions described, such neutralization should not be necessary in order to use the reaction product for PTT manufacture.
The purity of the cyclic dimer which has been removed from the polymerization mixture, generally ranges from about 10 percent to about 100 percent by weight pure, usually from about 50 to about 95 percent by weight pure. The remainder is typically PTT polymer and oligomers. The above process steps convert the PTT polymer and oligomers, at least in part, to derivatives soluble in PDO and are also recycled into the polymerization process.
The reaction mixture may be recycled into the polymerization process at any stage prior to the polycondensation step. It is preferred that it be added to the feed, most preferably the PDO feed or a PDO/TPA paste feed. It can also be added during esterification.